1. Field of the Invention
The present invention relates to a multi channel ADPCM (Adaptive Differential Pulse Code Modulation) compression and expansion device, and, more specifically, relates to a multi channel ADPCM compression and expansion device which allows simultaneous calls in multi channels as well as allows to be formed into an one-chip IC.
The multi channel ADPCM compression and expansion device referred to in the present invention is built-in such as in a main body (parent unit) of a digital cordless telephone set, a base station for telephone communication and a terminal device for cordless telephone, is designed to operate, for example, to compress digital signals of speech voice and transfer the compressed signals to a child unit connected in cordless, to expand like digital signals sent out from the child unit and transfer the expanded signals to the parent unit in it, and then to send out to public telecommunication network such as ISDN, and is to be applied to a so called second generation digital cordless telephone system which allows calls in cordless with other parties in homes, offices and outdoors by making use of digital signals.
2. Background Art
For telephone communication from moving bodies such as car equipped telephone and portable telephone, signal transmission and reception between one parent station and many child stations is performed via TDMA (Time Division Multi Access) control by compressing voice digital data. In the TDMA control the signal transmission and reception is performed by allotting for a child station a predetermined communication period and data amount, for example, in one of the standards of the TDMA control methods "RCR STD-28", section time width of 5 ms and data amount of 160 bits are allotted for one slot voice data which is a unit of signal transmission and reception in one child station.
When analog voice signals are digitized in 4 bit conversion accuracy according to the above standard by making use of sampling frequency of 8 kHz for signal transmission and reception processing of one slot, voice signal sampling of 40 times can be performed in one slot.
By making use of such TDMA control method a second generation digital cordless telephone set using TDD (Time Division Duplex) system which allows the simultaneous calls such as for homes is now under investigation. For realizing such system, a multi channel, in particular, 4 channel or the like ADPCM compression and expansion device is necessitated which allows digital signal transmission and reception between TDMA control devices having the TDD control function. Such multi channel ADPCM compression and expansion device is to be built-in in such devices as a main body cordless telephone set, a base station for telephone communication and a terminal device which are connected a public telecommunication network performing signal transmission and reception via digital signals. The multi channel ADPCM compression and expansion device is designed to perform a bidirectional conversion, in that, to compress PCM signals of voice digital data received from the public network, to convert the compressed signal into ADPCM signals and to send the same to a TDMA control device operating under TDD method, and conversely to receive ADPCM signals from a child unit via a TDMA control device, to expand the received signal to restore the same into PCM signals and send out the same to the public network operating in digital signals.
In this simultaneous call system, the signal transmission and reception between a parent unit and a child unit are performed in cordless by making use of compressed ADPCM signals while being divided by time sharing manner via the TDMA control device and the belonging circuits such as a radio equipment provided, for example, in a parent unit. In the child unit, the ADPCM signals are restored into PCM signals and then the restored PCM signals are demodulated into voice. When transmitting signals from the child unit, voice signals are converted into PCM signals and the PCM signals are further compressed and converted into ADPCM signals and are sent out in cordless to the TDMA control device and belonging circuits provided in the parent unit. The TDMA control device and the belonging circuit expand the received data from the child unit into PCM signals and send out the same to the public network.
Through the provision of an ADPCM processing circuit which can reduce transmission signal bit number in a midway of signal transmission and reception passage in a digital telephone system as explained above, signal data transmission and reception of a greater amount can be achieved in a limited period as well as simultaneous calls in time sharing manner can be realized.
The ADPCM signal conversion device is conventionally known which compresses data and converts into PCM signals and further converts from the PCM signals into ADPCM signals. However, such conventional device is for one channel use in which transmitter side voice signals are compressed and coded in a form of ADPCM signals then transmitted, and in which receiver side the ADPCM signals is expanded and demodulated into voice signals. When applying such conventional ADPCM conversion device in a digital cordless telephone system as it is, a plurality of ADPCM conversion devices have to be provided in parallel.
However, in an actual application, even when a plurality of the devices are provided in parallel, the one channel in the respective devices is insufficient. A plurality of channels of about 4 channels are necessary for outdoor base stations and private base stations, and further for ordinary home use telephone sets which use many child units about 4 channels will be necessitated. It is preferable to provide as many channels as possible. Therefore, in the above instance ADPCM conversion devices covering 4 channels are provided and are multi-controlled of which entire circuit structure becomes excessively large scale. On one hand, a parent unit used in such as homes is required to be no such large scaled device as well as to be low price. Therefore, it is required to constitute the circuit as simple as possible, and possibly it is desired to constitute the circuit which allows to be formed into one-chip IC. However, it is more difficult to form the circuit using the conventional ADPCM conversion devices for multi channels into one-chip IC based on the current technology, because mutual control between respective channels within one signal transmission slot has to be performed.